Fuses ratings

[Mike O'Dell]

fast and precise response is an advantage of the
"electronic fuses" now being used in "multiplexed
power systems".  the name is misleading because
it is the *control system* which is multiplexed.
the systems are based on the idea of a large DC
bus running the length of a boat or RV with taps
feeding electronic switching hubs. the drops for
switched loads come off those hubs instead of
doing the home-run back to a central distribution panel.
the system is controlled by a bus system - usually
some flavor of RS-485 - which has control panels
placed wherever they are wanted.

the power switching hubs can be programmed so that
each channel (varies from 2-16 in a hub) listens to
one or several switches (for N-way control), and
the overcurrent level and response curve for each 
channel is likewise programmable up to some maximum 
for the channel (usually enforced with a fuse, breaker, 
or polyfuse).

some of the hubs have manual over-rides on circuits
so they can be switched even if there is a total
failure of the control system.

why add all this complexity just for switching
DC loads?  the first answer is "flexibility" to
move switches around and add additional control
stations very easily, but the *real* reason is
WEIGHT. a 60 foot bus of a 00 cable pair weights
appreciably less than all the home-run tails that
would be missing by just running to a nearby hub
on the centerline. This is a 12V or 24V system
and I-squared-R is *not* your friend and most of
those home-runs are (duplex or triplex) #12 or maybe #14 
(and the wiring standards prohibit anything smaller than #16
for anything but very low current control signals).

in my boat, we had to freeze the design before
the buss-network switching systems were baked enough
to bet on, so we had to do the home-run thing mostly
(small sub-panel forward to take care of cabin hotel loads).
If we had it to do over and could use a buss-net system,
the electrician that did the wiring estimates that 
we could save at least 3000 pounds of copper,
not to mention the *cost* of a ton-and-a-half of copper.

as you might guess, airliners were the first applications
of this technology and it has existed for some time in the
"why use lead when gold will do" world of commercial aviation.
recently with the explosive growth in power semiconductors
and the drop in cost of very cheap micro-controllers, it is
now easy to put a spring-head in every switch bank and
every power switching pod and take advantage of the savings.

      -mo